(1) Field of the Invention
This invention relates generally to a process and apparatus for producing high purity alkali metal hydroxide in an electrolytic cell.
More specifically this invention relates to a process and apparatus for production of concentrated alkali metal hydroxide substantially free of alkali metal halides and other impurities in an electrolytic cell wherein a stable selectively permeable hydrated membrane is interposed between a dimensionally stable foraminous anode and a cathode to form individual anolyte and catholyte compartments.
(2) State of the Art
Concentrated alkali metal hydroxide solutions have previously been prepared by the electrolysis of alkali metal halide solutions in mercury type electrolytic cells, such cells frequently being referred to as flowing cathode mercury cells. The alkali metal hydroxide aqueous solution produced in such cells is generally of high concentration for example about 50 to 73 weight percent alkali metal hydroxide, and substantially free of alkali metal halide. A produce with these characteristics directly satisfies the requirements for various industrial applications. However, the mercury type cell currently has several disadvantages the major one being the pollution of streams caused by the effluent of said cells. This discharge has created a mercury pollution problem in the environment whenever such cells are in use. Although extensive efforts have been made to control the amount of mercury pollution caused by the effluent from such cells, it is generally considered that the complete elimination of the pollution of water and soil is virtually impossible. Because of the current objections to any type of pollution and the very strict governmental regulatory provisions proposed to control all types of pollution, there is the imminent possibility that such cells will not be tolerated for more than a few years and soon will become obsolete. Even if improved methods of preventing mercury pollution by the effluent of said cells are found and introduced, and even if governmental regulatory provisions can be met and the cells remain in use, they suffer the additional disadvantages of being expensive and complex, and of frequently causing erratic operating conditions. Also cell operators are constantly exposed to toxic hazards. Large quantities of mercury are required per cell and the market price of mercury is generally high. In addition a certain quantity of the mercury utilized in the normal operation of such cells is irretrievably lost in the effluent streams regardless of the rigid recovery techniques employed to reclaim the mercury from the amalgam formed in the cell.
Diaphragm cells are also known for producing alkali metal hydroxide solutions electrolytically. In this type of cell a porous cathode with an overlying porous diaphragm is used to separate or to serve as a barrier between the catholyte and anolyte compartments of the cell. An objectionable feature of this cell in the electrolysis of alkali metal halide is the porosity of the diaphragm which, although it serves to separate the cell into anode and cathode compartments, permits the aqueous electrolyte solution to be unselectively transported into the cathode compartment. Because of the water content of the electrolyte solution the concentrations of the alkali metal hydroxide product are limited to from about 12 to about 18 weight percent. Another disadvantage of this cell is the tendency of the hydroxyl ion formed in the cathode compartment to migrate back through the porous cathode and diaphragm because of electromigration and diffusion. This so called back-migration results in undesirable side products or impurities and a loss of operating current efficiency in the cell because of the additional current consumed by the cathode-anode anion migration. Prior art efforts to overcome the disadvantage of hydroxyl ion back-migration have resulted in forcing a flow of the alkali metal halide electrolyte through the porous diaphragm by positive means such as hydraulic flow and electro-osmotic pumping. This technique is referred to as the percolating diaphragm method. This type of cell operation results in not only a limitation of the concentration of the alkali metal hydroxide product, since the water content of the circulating aqueous electrolyte solution has a dilution effect and prevents concentration of the hydroxide, but also in retention of the impurities present in the brine initially charged to the cell. Although the alkali metal hydroxide solutions obtained with the diaphragm-type cell may be concentrated to meet higher concentration requirements, the evaporation and purification techniques required are time consuming, inefficient and objectionably expensive.
In order to overcome the disadvantages of both the mercury and diaphragm-type cells, membrane-type cells have been proposed for producing alkali metal hydroxides. The permselective membranes used in these cells are referred to as cationic since they permit the passage of positively charged ions. They are generally made from cation exchange resins, usually ionogenic particles embedded or grafted into a fiber matrix or carrier. At low caustic concentrations such a cell design limits the back migration of negatively charged hydroxyl ions and slows down the passage of water so that moderately high concentrations of solutions of alkali metal hydroxide are formed in the cathode compartment, however, these cells require the addition of water to the catholyte which causes lower current efficiency. Such membranes are disclosed in U.S. Pat. No. 2,967,807, where their use in the production of alkali metal hydroxide solutions is also shown. Various other permselective and so-called diaphragm-type membranes have been proposed in the prior art and such cationic membranes have solved the problem of halide ion exclusion and to some extent overcome the problem of the back migration of the hydroxyl ions of the porous diaphragm cells as well as the inclusion of objectionable impurities particularly alkali metal halide, in the resultant product of such cells. However, the proposed membrane cells also have limitations which have prevented their wide-spread use such as lower current efficiency, structural degradation, low product concentration, high voltage and reduced operating temperature requirements. The membranes are subject to degradation by the corrosive nature of the chemicals of the cells such as chlorine, caustic and hypochlorite and are also degraded by higher operating temperatures over rather short periods of time. For example, such prior membranes deteriorated after less than one thousand hours of continuous operation. The increased expenses due to the frequent replacement of such membranes has detracted from their use in obtaining improved results over the porous and percolating diaphragm-type cell. The low current efficiencies found when the previous membrane cells were used were caused by hydroxyl ion back-migration to the anode from the cathode chamber and its subsequent oxidation at the anode surface. The low voltage efficiencies were caused by the low permeabilities and heterogeneous gel characteristics of these membrane materials. Also high concentrations of alkali metal hydroxide on the order of 50-56 percent are unobtainable with the use of the previous membrane cells, maximum product concentrations of only about 20 to about 40 percent have been previously produced under optimum conditions.